(1) Field of the Invention
This invention relates to a photosensitive element for forming a mask useful to prepare a series of masking plates, each plate for a different color, said plates being used to prepare a printing plate used in multicolor printing.
(2) Description of the Prior Art
When an image is printed not in screen dot but in solid, the prior art has employed a process utilizing a manuscript composed by surrounding the image to be reproduced with a so called enclosing line (said manuscript being hereinafter referred to as "enclosing line manuscript"), wherein the masking plate used for making the printing plate is prepared by subjecting the exposed photosensitive material to a photographic treatment. That is, a negative or positive enclosing line manuscript is printed on a light shielding photosensitive masking layer of the photosensitive material, which is then developed or developed and etched, depending on the type of masking layer. By this photographic treatment, as shown in FIG. 1A, there is formed on a transparent film 2 of a photosensitive material 1, a relief image 6 in the photosensitive masking layer where ditches 5 corresponding to enclosing lines 4 of the manuscript 3 are formed. Next, as shown in FIG. 1B, when the relief image 6 is peeled off from the transparent film 2 with a needle, knife or the like, leaving masking parts 6', there is obtained a masking plate 8 having a desired image as concave parts 7 providing light shielding using the masking plate 8 thus obtained, a similar photographic treatment makes a plate surface.
In the case of multicolor printing, a masking plate for each color is usually prepared by subjecting the relief image which has been obtained from a negative manuscript to selective peeling of areas for a desired color. The negative manuscript contains enclosing lines which are unnecessary for a masking plate for a certain color but required for a masking plate for another color. Since groves (ditches) which are formed upon developing in the photosensitive masking layer corresponding to the enclosing lines do not shield light, it is necessary when preparing the masking plate for each color by selective peeling, that the ditches corresponding to the unnecessary enclosing lines be filled up and retouched by applying on the image a coating liquid with a high light shielding property which is called a correction liquid retouching fluid and drying the retouching fluid applied on the image. The same operation is required for letters, etc. corresponding to a line itself without surrounding them with enclosing lines.
In printing where different colors are butted without superposition, ditches corresponding to enclosing lines give one of the colors and a masking plate for another color is prepared by selective peeling-off after the retouching fluid is coated on the whole surface of the image corresponding to the parts to be butted and then dried to fill up and retouch the ditches.
As the retouching fluid used for such a purpose, both organic solvent types and aqueous solution types are used. In the case of an organic solvent type, care should be taken in its handling because of its inflammability and undesirable odor. The aqueous solution type has some defects such as lower operation efficiency due to its very slow drying.
In addition, a wide flat area and a uniform coating technique are required for coating with retouching fluid, a step begging improvement in the plate making process.